How do i reject an upload too big or already exist ( ESP8266WebServer )

[jbreizh]

Hello,
i am working on a light painting app and i use ajax request to upload bitmap on my ESP8266. If you want to see my complete code, check my repo : imagepainting2 in my working version and imagepainting3 my advance version.
My ajax client request :

//--------------------------------------------------
function requestFileUpload(form) {
    var xhr = new XMLHttpRequest();
    xhr.onload = function() {
        if (this.status == 200) {
            updateStatus(this.responseText, "green");
        } else {
            updateStatus("UPLOAD ERROR : UPLOAD FAILED", "red");
        }
        requestSystemRead();
    };
    
    xhr.upload.onprogress = function(evt) {
        var percentComplete = Math.floor(evt.loaded / evt.total * 100);
        updateStatus("UPLOAD PROGRESS :" + percentComplete + "%", "orange");
    };
    
    xhr.onerror = function() {
        updateStatus("UPLOAD ERROR : CONNECTION LOST", "red");
    };
    
    xhr.open("POST", address + "/upload", true);
    xhr.send(form);
}

My ESP8266 server part :

 // handle file upload
  server.on("/upload", HTTP_POST, []() {
    server.send(200, "text/plain", "UPLOAD SUCCESS");
  }, handleFileUpload);

void handleFileUpload()
{
  //
  HTTPUpload& upload = server.upload();

  //
  FSInfo fs_info;
  LittleFS.info(fs_info);
  if (upload.contentLength > (fs_info.totalBytes-fs_info.usedBytes))
  {
    Serial.println("Coucou");
    server.send(413, "text/plain", "UPLOAD ERROR : TOO BIG");
    return;
  }

  // Upload start
  if (upload.status == UPLOAD_FILE_START)
  {
    // Retrieve and correct filname
    String filename = upload.filename;
    if (!filename.startsWith("/")) filename = "/" + filename;

    // Open the file for writing in LittleFS (create if it doesn't exist)
    UPLOADFILE = LittleFS.open(filename, "w");
  }

  // Upload in progress
  else if (upload.status == UPLOAD_FILE_WRITE)
  {
    //Write the received bytes to the file
    if (UPLOADFILE) UPLOADFILE.write(upload.buf, upload.currentSize);
  }

  // Upload end
  else if (upload.status == UPLOAD_FILE_END)
  {
    //Close the file
    if (UPLOADFILE) UPLOADFILE.close();
  }
}

You can see that in my code, i try to reject files that are too big. It work but the client still send the file, it's just the server that doesn't "record" it.
So my question is : how to reject nicely the upload if the file is to big (or other condition) ?
Thanks for you time !!